Method of treating by-product salt yieled with polyarylene sulfide

ABSTRACT

There is disclosed a process for treating a by-product salt of a polyarylene sulfide which comprises subjecting solid matter containing an aprotic organic solvent and the by-product salt that is formed in the case of producing a polyarylene sulfide by polymerization condensation reaction of an alkali metal sulfide and a dihalogenated aromatic compound in the aprotic organic solvent to a drying treatment followed by a firing treatment of the resultant dryingly treated matter at a temperature in the range of 500 to 1000° C. The above process is capable of enhancing the solubility in water, of the by-product salt comprising sodium chloride as the principal component in the production of polyarylene sulfide, and affording waste water which is lowered in SS and COD in the case where the by-product salt is dissolved in water and is discharged as waste water.

TECHNICAL FIELD

[0001] The present invention relates to a process for treating aby-product salt that is formed as a by-product in the production of apolyarylene sulfide (hereinafter sometimes referred to as “PAS”). Moreparticularly, it pertains to a process for treating a by-product saltwhich is capable of enhancing the solubility in water, of the by-productsalt that is formed as a by-product in the production of the polyarylenesulfide, and affording waste water which is lowered in SS (suspendedsolid particles components) and COD (chemical oxygen demand) in the casewhere the by-product salt is dissolved in water and is discharged aswaste water.

BACKGROUND ART

[0002] A polyarylene sulfide, especially polyphenylene sulfide is knownas an engineering plastic which is excellent in mechanical strength,heat resistance and the like and which has good electricalcharacteristics and high rigidity. Thus it is widely employed as avariety of materials such as electronic machinery parts and electricalmachinery parts.

[0003] As a process for producing a polyarylene sulfide, there hasheretofore been employed in general, a process in which a dihalogenatedaromatic compound such as p-dichlorobenzene and a sodium salt such assodium sulfide are reacted with each other in the presence of an aproticorganic solvent such as N-methyl-2-pyrrolidone (hereinafter sometimesabbreviated to “NMP”) to effect a polymerization condensation reaction.

[0004] In the above-mentioned process however, a sodium halogenide as aby-product, which is insoluble in a solvent such as NMP, is incorporatedin a resin, whereby the removal of the by-product by cleaning has beenfar from easy. Moreover, it has been extremely difficult to continuouslytreat at a high temperature, a polymer and sodium halogenide as aby-product that are formed in the aforesaid process.

[0005] Under such circumstances, it has been found that continuoustreatment of the polymer at a high temperature is made possible by amethod in which the polymerization condensation reaction is conducted byusing a lithium salt in place of the sodium salt in the presence of NMPas a solvent to form a lithium halogenide, since the lithium halogenideis soluble in a number of aprotic organic solvents such as NMP as apolymerization solvent, thus enabling it comparatively easy to lower theconcentration of lithium in the resin (refer to Japanese PatentApplication Laid-Open No. 207027/1995 (Heisei 7).

[0006] In the above-mentioned process however, lithium sulfide as astarting raw material for polymerization reaction is obtained, forinstance, by reacting lithium chloride with sodium hydroxide to obtainlithium hydroxide, reacting the resultant lithium hydroxide withhydrogen sulfide to obtain lithium hydrosulfide, and subjecting theresultant lithium hydrosulfide to hydrogen sulfide removing reaction. Insuch reaction system, when lithium chloride is reacted with sodiumhydroxide, sodium chloride is formed as a reaction by-product. Theresultant sodium chloride, when being subjected to polymerizationreaction as such, gives rise to a problem same as the problem arisingfrom the foregoing sodium salt. Accordingly, it is customary that slurrysolution containing lithium hydrosulfide thus formed is subjected tosolid-liquid separation to separate it into a solvent portion containinglithium hydrosulfide and solid matter containing a by-product salt suchas sodium chloride, and the solvent portion is subjected to next lithiumsulfide synthesis step, while the by-product salt such as sodiumchloride is subjected to drying treatment and is thrown into thediscard.

[0007] In an ordinary synthesis system, sodium chloride is readilydissolved in water and can be discharged outside the system as wastewater. However, in the case of recycling the solvent phase or the likewhich is obtained by solid-liquid separation of PAS polymerizationreaction mixture to use as a starting material, there exists a smallamount of an oligomer in the solid mixture. In this case, there iscaused a problem in that the by-product salt such as sodium chloridewhich is formed in the aforesaid reaction and subjected to dryingtreatment is markedly inferior in solubility in water, thus takes a longtime to be dissolved in water and besides, the SS and COD in water afterdissolution are made high, thereby inevitably increasing the waste watertreatment expense.

[0008] It is thought that the surface of sodium chloride particles arecovered by the PAS oligomer contained in the starting liquid materialowing to the drying treatment, and thus the solubility of such sodiumchloride in water is worsened, and that in the case where the dryinglytreated matter is dissolved in water to form waste water, the SS and CODare unreasonably increased by the PAS oligomer, residual solvent, etc.in water.

DISCLOSURE OF THE INVENTION

[0009] Under such circumstances, it is a general object of the presentinvention to provide a process for treating a by-product salt ofpolyarylene sulfide which procss is capable of enhancing the solubilityin water, of the by-product salt which comprises sodium chloride as aprincipal component in the production of the polyarylene sulfide, andaffording waste water which is lowered in SS and COD in the case wherethe by-product salt is dissolved in water and discharged outside thesystem as waste water.

[0010] In such circumstances, intensive extensive research andinvestigation were accumulated by the present inventors in order toachieve the above-mentioned objects. As a result, it has been found thatthe objects of the present invention can be achieved by dryinglytreating a solid mixture containing an aprotic organic solvent and aby-product salt, followed by a firing treatment of the solid mixture,said salt being by-poduced in the case of producing a polyarylenesulfide from an alkali metal sulfide and a dihalogenated aromaticcompound in the aprotic organic solvent. The present invention has beenaccomplished on the basis of the foregoing findings and information.Specifically, the present invention provides a process for treating aby-product salt from a polyarylene sulfide which comprises subjectingsolid matter containing an aprotic organic solvent and the by-productsalt that is formed in the case of producing a polyarylene sulfide bypolymerization condensation reaction of an alkali metal sulfide and adihalogenated aromatic compound in the aprotic organic solvent to dryingtreatment, followed by a firing treatment of the resultant dryinglytreated matter at a temperature in the range of 500 to 1000° C.

THE MOST PREFERRED EMBODIMENTS TO CARRY OUT THE INVENTION

[0011] In the following, more detailed description will be given of thepresent invention.

[0012] The polymerization condensation reaction between an alkali metalsulfide and a dihalogenated aromatic compound is put into practice in anaprotic organic solvent.

[0013] Examples of the aprotic organic solvent include in general,aprotic organic polar solvents such as amide compounds, lactamcompounds, urea compounds, organosulfur compounds and cyclicorganophosphorus compounds.

[0014] The above-mentioned amide compounds among aprotic organic polarsolvents are exemplified by N,N-dimethylformamide; N,N-diethylformamide;N,N-dimethylacetoamide; N,N-diethylacetoamide; N,N-dipropylacetoamide;N,N-dimethylbenzoic acid amide, etc.

[0015] The aforesaid lactam compounds are exemplified byN-alkylcaprolactam such as caprolactam; N-methylcaprolactam;N-ethylcaprolactam; N-isopropylcaprolactam; N-isobutylcaprolactam;N-n-propylcaprolactam; N-n-butylcaprolactam; andN-cyclohexylcaprolactam; N-methyl-2-pyrrolidone (NMP);N-ethyl-2-pyrrolidone; N-isopropyl-2-pyrrolidone;N-isobutyl-2-pyrrolidone; N-n-propyl-2-pyrrolidone;N-n-butyl-2-pyrrolidone; N-cyclohexyl-2-pyrrolidone;N-methyl-3-methyl-2-pyrrolidone; N-ethyl-3-methyl-2-pyrrolidone;N-methyl-3,4,5-trimethyl-2-pyrrolidone; N-methyl-2-piperidone;N-ethyl-2-piperidone; N-isopropyl-2-piperidone;N-methyl-6-methyl-2-piperidone; N-methyl-3-ethyl-2-piperidone, etc.

[0016] The aforesaid urea compounds are exemplified by tetramethylurea;N,N′-dimethylethyleneurea; N,N′-dimethylpropyleneurea, etc.

[0017] The aforesaid organosulfur compounds are exemplified bydimethylsulfoxide; diethylsulfoxide; diphenylsulfone;1-methyl-1-oxosulfolane; 1-ethyl-1-oxosulfolane; 1-phenyl-1-oxosulfolaneetc.

[0018] The aforesaid cyclic organophosphorus compounds are exemplifiedby 1-methyl-1-oxophosfolane; 1-n-propyl-1-oxophosfolane;1-phenyl-1-oxophosfolane, etc.

[0019] Any of the above-exemplified aprotic organic polar solvent can beused alone or by mixing with at least one other or by mixing with asolvent which is not cited above and does not impair the object of thepresent invention so as to enable the mixture to be used as theforegoing aprotic organic solvent. Of the various aprotic organicsolvents as exemplified above are preferable N-alkylcaprolactam andN-alkylpyrrolidone, among which N-methyl-2-pyrrolidone (NMP) isparticularly preferable.

[0020] The polyarylene sulfide mentioned in the present invention is,for instance, a polymer having at least 70 mol % of the repeating unitrepresented by —Ar—S— wherein Ar is an arylene group. A typical exampleincludes an polyarylene sulfide having at least 70 mol % of therepeating unit represented by the following general formula (I):

[0021] wherein R¹ is a substituent selected from an alkyl group havingat most 6 carbon atoms, an alkoxy group, a phenyl group, a metal salt ofa carboxylic acid, an amino group, a nitro group and a halogen atom suchas a fluorine atom, chlorine atom and bromine atom; m is an integer from0 to 4; and n denotes average degree of polymerization, and ranges from10 to 200.

[0022] The known polyarylene sulfide is generally classified by itsproduction process into that having a substantially linear molecularstructure without a branched or crosslinked structure, and that having abranched or crosslinked structure. In the production process relating tothe present invention, any of the polyarylene sulfide is effectivelyusable.

[0023] The polyarylene sulfide is exemplified by a homopolymer or acopolymer each having as the repeating unit, at least 70 mol %,preferably 80 mol % of p-phenylene sulfide. Examples of constitutingunit of the copolymer include m-phenylene sulfide unit; o-phenylenesulfide unit; p, p′-diphenyleneketone sulfide unit; p,p′-diphenylenesulfone sulfide unit; p, p′-biphenylene sulfide unit; p,p′-diphenylene ether sulfide unit; p, p′-di-phenylenemethylene sulfideunit; p, p′-diphenylenecumenyl sulfide unit; and naphthylene sulfideunit. Moreover, as a polyarylene sulfide that can be an object relatingto the present invention, mention can be made in addition to theforegoing substantially linear polymer, of a branched or crosslinkedpolyarylene sulfide in which a small amount of monomer having at leastthree functional groups as a part of monomers is polymerized, and ablended polymer in which the polyarylene sulfide just cited is blendedwith the foregoing substantially linear polymer.

[0024] In the above-mentioned polymerization condensation reaction useis made as starting raw materials, of an alkali metal sulfide and adihalogenated aromatic compound. Examples of the dihalogenated aromaticcompound include dihalogenated benzene such as m-dihalogenated benzeneand p-dihalogenated benzene, alkyl-substituted dihalogenated benzene orcycloalkyl-substituted dihalogenated benzene such as 2,3-dihalogenatedtoluene; 2,5-dihalogenated toluene; 2,6-dihalogenated toluene;3,4-dihalogenated toluene; 2,5-dihalogenated xylene;1-ethyl-2,5-dihalogenated benzene; 1,2,4,5-tetramethyl-3,6-dihalogenatedbenzene; 1-n-hexyl-2,5-dihalogenated benzene; and1-cyclohexy-2,5-dihalogenated benzene, aryl-substituted dihalogenatedbenzene such as 1-phenyl-2,5-dihalogenated benzene;1-benzyl-2,5-dihalogenated benzene; and 1-p-toluyl-2,5-dihalogenatedbenzene, dihalobiphenyl such as 4,4′-dihalobiphenyl, dihalogenatednaphthalene such as 1,4-dihalonaphthalene; 1,6-dihalonaphthalene; and2,6-dihalonaphthalene, and the like.

[0025] On the other hand, examples of the alkali metal sulfide includesodium sulfide, lithium sulfide and potassium sulfide, of which each maybe used alone or in combination with at least one other. Further, thealkali metal sulfide may be used in combination with an alkaline earthmetal sulfide and/or an other sulfur source. In the process according tothe present invention lithium sulfide is preferably used in particular.For instance, lithium sulfide can be produced by reacting lithiumhydroxide and hydrogen sulfide in an aprotic organic solvent to formlithium hydrosulfide, and subsequently dehydrogensulfiding the resultantreaction liquid.

[0026] That is to say, lithium sulfide is synthesized by thedehydrogensulfiding reaction of lithium hydrosulfide according to thefollowing chemical equation.

2 LiOH+H₂S→LiSH+H₂O

2 LiSH→Li₂S+H₂S

[0027] The lithium hydroxide to be used therein is obtained by reactingsodium hydroxide with lithium chloride. The present invention relates toa process for effectively treating a by-product salt such as sodiumchloride which is formed in this reaction. The lithium chloride to beused as a starting raw material in the aforesaid reaction may be newlyadded in whole amount, but it is possible to use lithium chloridecontained in a solvent phase which is obtained by solid-liquidseparation of PAS polymerization reaction mixture as it is or byproperly refining the same.

[0028] The hydrogen sulfide is not specifically limited, but there isusable, for instance, hydrogen sulfide in an off gas which is generatedfrom a petroleum refinery.

[0029] The molar ratio of hydrogen sulfide to lithium hydroxide that areto be used in the treatment (molar ratio of hydrogen sulfide/lithiumhydroxide) is usually at least 1.0 (mol/mol), especially at least 1.05(mol/mol).

[0030] In carrying out the above-mentioned reaction, for instance, areaction vessel is charged inside with the foregoing aprotic organicsolvent and lithium hydroxide, and hydrogen sulfide is blown into theresultant charged liquid to react therewith. In this case, the reactionmay be put into practice by blowing hydrogen sulfide in advance into theaprotic organic solvent to dissolve the same, and mixing lithiumhydroxide with the solvent thus prepared.

[0031] In the present invention, hydrogen sulfide may be blown atatmospheric pressure or under pressure. The blowing time and blowingvelocity are not specifically limited. A method for blowing hydrogensulfide is not specifically limited, but may be for instance, aconventionally used method in which an aprotic organic solvent isincorporated with lithium hydroxide under stirring, and further withgaseous hydrogen sulfide by means of bubbling. This method can becarried out in a continuous system in the absence or presence of water.The reaction temperature is in the range of preferably 80 to 120° C.,particularly preferably 90 to 110° C.

[0032] By feeding hydrogen sulfide in such a manner, the lithiumhydroxide that had been present in solid form in the system is convertedinto lithium hydrosulfide, which is dissolved in the liquid portion ofthe system.

[0033] In the present invention, after the formation of lithiumhydrosulfide, the slurry containing the lithium hydrosulfide and aby-product salt such as sodium chloride is subjected to solid-liquidseparation by means of centrifugal separation, whereby the slurry isseparated into a solvent phase containing lithium hydrosulfide and aby-product salt such as sodium chloride.

[0034] The amounts of the by-product salt such as sodium chloride andthe aprotic organic solvent in the solid mixture depend upon a varietyof conditions such as the types of the starting raw materials and thesolvent, reaction conditions and solid-liquid separation conditions. Ingeneral, the contents of sodium chloride and the aprotic organic solventare 40 to 60% by weight, and 60 to 40% by weight, respectively. Theparticle diameter of sodium chloride in solid form is approximately 5 to10 micrometer.

[0035] The solvent portion containing lithium hydrosulfide that has beenseparated by solid-liquid separation is utilized for synthesis reactionin the next step, and the solid mixture is dried to remove and discardthe by-product such as solid sodium chloride contained therein. Theaprotic organic solvent that has been evaporated and separated throughthe drying is recovered and reused by being recycled through thesynthesis step for lithium hydrosulfide. The drying efficiency can beenhanced by rinsing, prior to the drying, the aprotic organic solventwhich contains polymerized by-products such as PAS oligomer stuck tosodium chloride and the like with a refined aprotic organic solvent.

[0036] In the present invention, the aforesaid solid matter is at firstsubjected to a drying treatment in order to recover the organic solventcontained therein. The drying treatment may be any of batchwise systemand continuous system by means of atmospheric or vacuum drying. The kindof a dryer is not specifically limited, but there may be used a dryersuch as a conventionally used disc dryer, a dryer having a self-cleaningproperty and the like.

[0037] As the conditions in the above-mentioned drying treatment, thetemperature of the object to be dried is in the range of 120 to 250° C.,preferably 140 to 220° C., and the pressure thereof is atmosphericpressure to 0.001 MPa, preferably atmospheric pressure to 0.01 MPa. Thedrying treatment time, which depends upon the temperature and pressureof the drying treatment, type of the dryer and the like, can not beunequivocally determined, but one to 10 hours is generally sufficient.

[0038] In the present invention, the reaction solvent is recovered bysubjecting the solid matter to the drying treatment in such manner, andthen the solid matter thus dried is subjected to a firing treatment at atemperature in the range of 500 to 1000° C. By the firing treatment,such organic matter as arylene sulfide oligomer and the solvent areremoved, and hence the firingly treated matter is enhanced in solubilityin water, and when dissolved in water to form waste water, SS and CODtherein are markedly lowered.

[0039] As the method for the firing treatment, which may be any ofbatchwise and continuous systems, a solid firing method or meltingmethod is preferably adoptable. In the case where the solid firingmethod is adopted, the firing treatment is carried out usually by usinga kiln at a temperature preferably in the range of 500 to 800° C. forabout 0.5 to 5 hours. On the other hand, in the case where the meltingmethod is adopted, the firing treatment is carried out usually by usinga melting furnace at a temperature preferably in the range of 800 to1000° C. for about 0.1 to 2 hours.

[0040] In the present invention, a comonomer, a branching agent, an endterminator and the like may be combinedly used with the above-mentioneddihalogenated aromatic compound to the extent that the effect thereof isnot impaired thereby. Examples of the comonomer include2,3-dichlorophenol; 2,3-dibromophenol; 2,4-di-chlorophenol;2,4-dibromophenol; 2,5-dichlorophenol; 2,5-dibromophenol;2,4-dichloroaniline; 2,4-dibromoaniline; 2,5-dichloroaniline;2,5-dibromoaniline; 3,3′-dichloro-4,4′-diaminobiphenyl;3,3′-dibromo-4,4′-diaminobiphenyl; 3,3′-dichloro-4,4′-dihydoxybiphenyl;3,3′-dibromo-4,4′-dihydoxybiphenyl; di(3-chloro-4-amino)phenylmethane;m-dichlorobenzene; m-dibromobenzene; o-dichlorobenzene;o-dibromobenzene; 4,4′-dichlorodiphenyl ether; and4,4′-dichlorodiphenylsulfone. Examples of the branching agent include1,2,4-trichlorobenzene; 1,3,5-trichlorobenzene; and1,2,3-trichlorobenzene.

[0041] Examples of the end terminator include a halogenated phenol suchas p-bromophenol; m-bromophenol; o-bromophenol; p-chlorophenol;m-chlorophenol; o-chlorophenol; p-fluorophenol; m-fluorophenol;o-fluorophenol; p-iodophenol; m-iodophenol; o-iodophenol. Of these,p-bromophenol and p-chlorophenol are preferable.

[0042] With regard to the proportion of the starting raw materials to beused in the process according to the present invention, the molar ratioof the dihalogenated aromatic compound to the alkali metal sulfide ispreferably 0.8 to 1.2, more preferably 0.9 to 1.1, particularlypreferably 0.95 to 1.05. When the molar ratio is outside the range of0.8 to 1.2, there exists a fear of failure to obtain a polyarylenesulfide having a high molecular weight.

[0043] The polymerization condensation reaction of PAS may be put intopractice by one stage reaction at a temperature in the range of 230 to290° C., approximately, preferably 240 to 280° C., more preferably 250to 270° C., or by the combination of the above-mentioned polymerizationcondensation and a preliminary polymerization prior thereto at atemperature in the range of 180 to 230° C., preferably 190 to 220° C.,more preferably 195 to 215° C. The polymerization condensation reactiontime is usually 0.5 to 10 hours, preferably 1.0 to 10 hours, morepreferably 1.5 to 10 hours. The polymerization condensation reactiontime, when being less than 0.5 hour, causes a fear of insufficientreaction resulting in failure to assure a sufficiently high molecularweight, whereas the reaction time, when being more than 10 hour, doesnot exert the working effect in proportion to the reaction time. Thepolymerization condensation vessel may be either a batchwise system or acontinuous system, but the latter is preferable in the presentinvention.

[0044] The present invention is particularly useful in the case ofrecyclingly using as a starting liquid material, the solvent phase whichis obtained by the solid-liquid separation for the polymerizationreaction mixture of PAS, that is, in the case where a small amount ofoligomer is present in the solid-state mixture.

[0045] In summarizing the working effects of the present invention, itis enabled by the process according to the present invention toremarkably enhance the solubility in water, of the by-product saltcomprising sodium chloride as a principal component, and when beingdissolved in water to form waste water, to provide a method for treatingthe by-product salt of polyarylene sulfide which method affords wastewater lowered in SS and COD in the case of producing a polyarylenesulfide.

[0046] In what follows, the present invention will be described in moredetail with reference to working examples, which however shall neverlimit the present invention thereto.

EXAMPLE 1

[0047] An aqueous solution of sodium hydroxide having a concentration of48% by weight in an amount of 118 kg was added to 545 kg of a startingraw material in liquid form which had been generated from a PASproduction pilot plant as polymer cleaning solution (chemicalcomposition: 11% by weight of LiCl, 0.6% by weight of PAS oligomer andthe balance being NMP). The slurry thus obtained was centrifuged toseparate into solid and liquid. As a result, there was obtained 233 kgof solid cake containing 14% by weight of LiOH and 35% by weight ofNaCl. The cake was added to 500 kg of NMP to be again slurried. In orderto convert the LiOH to LiSH, a sufficient amount of H₂S being 35 n-m 3in total was blown into the resultant slurry, while stirring the same.The resultant slurry containing LiSH was centrifuged. As a result, therewas obtained 158 kg of solid cake containing 51% by weight of NaCl. Thecake was added to 500 kg of NMP to be again slurried. The resultantslurry was centrifuged with a result that 154 kg of solid cakecontaining 52% by weight of NaCl was obtained. Subsequently, the solidcake was continuously dried by using a self-cleaning type dryermanufactured by Kurimoto Ltd. under the trade name “SC processor SCPseries 100” in the conditions including a temperature of the object tobe dried being 150° C., a heating medium temperature of 190° C.,pressure of 14 kPa-abs and a starting material feed rate of 38 kg/hour.The dried matter was further subjected to final batchwise drying at atemperature of the object to be dried being 200° C., a heating mediumtemperature of 240° C. under atmospheric pressure. The finally driedmatter had a chemical composition consisting of 2.0% by weight of PASoligomer, 710 ppm of NMP and the balance of NaCl.

[0048] The finally dried matter was fed in a continuous outside heatingtype kiln having an inside diameter of 150 mm and a length of 720 mm (aneffective volume of 12 liter) at a feed rate of 1.2 kg/hour, andsubsequently was subjected to a continuous firing treatment at 600° C.for 70 minutes as the retention time. The resultant firingly treatedmatter in an amount of 30 g was fed in 500 milliliter (mL) of water, anda measurement was made of the dissolving rate etc. thereof through thefollowing procedure. As a result, the treated matter indicated adissolving rate of at most 5 minutes, SS of 10 ppm and COD of 20 ppm.

[0049] {Measuring Method for Dissolving Rate}

[0050] By using an ion electrode meter which has already been immersedin water, the lapse of time from feeding the firingly treated matter inthe water under stirring with a stirrer until the ion potential thereofno longer rises is regarded as the dissolving rate (minutes) of theproduct.

EXAMPLE 2

[0051] The procedure in Example 1 was repeated to carry out the firingtreatment and evaluate the firingly treated matter except that thefiring temperature was set on 700° C. As a result, the treated matterindicated a dissolving rate of at most one minute, SS of 3 ppm and CODof 7 ppm.

EXAMPLE 3

[0052] The dried matter same as that in Example 1 was continuously fedin a swirling type melting furnace having an inside diameter of 300 mmand a length of 1500 mm (an effective volume of 90 liter) at a feed rateof 5.0 kg/hour, and subsequently was subjected to a continuous firingtreatment at 905° C. for 30 minutes as the retention time. The resultantfiringly treated matter was allowed to cool and evaluated in the samemanner as in Example 1. As a result, the treated matter indicated adissolving rate of at most one minute, while SS and COD were notdetected.

COMPARATIVE EXAMPLE 1

[0053] The dried matter same as that in Example 1 was evaluated in thesame manner as in Example 1 without firing treatment. As a result thedried matter indicated a dissolving rate of 30 minute, SS of 1200 ppmand COD of 212 ppm.

INDUSTRIAL APPLICABILITY

[0054] The present invention relates to a process for treating aby-product salt of polyarylene sulfide which process is capable ofenhancing the solubility in water, of the by-product salt that is formedas a by-product in producing polyarylene sulfide, and affording wastewater which is lowered in SS (suspended solid particles components) andCOD (chemical oxygen demand) in the case where the by-product salt isdissolved in water and is discharged as waste water. A polyarylenesulfide, especially polyphenylene sulfide is employed as an engineeringplastic which is excellent in mechanical strength, heat resistance andthe like and which has favorable electrical characteristics and highrigidity, and is useful as a variety of materials such as electronicmachinery parts and electrical machinery parts.

1. A process for treating a by-product salt of a polyarylene sulfidewhich comprises subjecting solid matter containing an aprotic organicsolvent and the by-product salt that is formed in the case of producinga polyarylene sulfide by polymerization condensation reaction of analkali metal sulfide and a dihalogenated aromatic compound in theaprotic organic solvent to a drying treatment, followed by a firingtreatment of the resultant dryingly treated matter at a temperature inthe range of 500 to 1000° C.
 2. The process according to claim 1,wherein the dryingly treated matter is subjected to the firing treatmentby means of solid firing method or melting method.
 3. The processaccording to claim 2, wherein the dryingly treated matter is subjectedto the firing treatment by means of solid firing method at a temperaturein the range of 500 to 800° C.
 4. The process according to claim 2,wherein the dryingly treated matter is subjected to the firing treatmentby means of melting method at a temperature in the range of 800 to 1000°C.
 5. The process according to claim 1, wherein the by-product salt issodium chloride.
 6. The process according to claim 1, wherein the alkalimetal sulfide is lithium sulfide.
 7. The process according to claim 6,wherein the lithium sulfide is produced by removing hydrogen sulfidefrom lithium hydrosulfide which is produced by reacting lithiumhydroxide with hydrogen sulfide in an aprotic organic solvent.
 8. Theprocess according to claim 7, wherein the lithium hydroxide is producedby reacting sodium hydroxide with lithium chloride.
 9. The processaccording to claim 1, wherein the polyarylene sulfide has at least 70mol % of the repeating unit represented by the following general formula(I):

wherein R¹ is a substituent selected from an alkyl group having at most6 carbon atoms, an alkoxy group, a phenyl group, a metal salt of acarboxylic acid, an amino group, a nitro group and a halogen atom; m isan integer from 0 to 4; and n denotes average degree of polymerizationand ranges from 10 to
 200. 10. The process according to claim 1, whereinthe aprotic organic solvent is N-methyl-2-pyrrolidone.